This issue with the Smith & Wesson 15-22 is not an isolated case. The article from The Firearms Blog indicates that there have been multiple incidents of issues. Enough so that they are banned from the Appleseed events. Ref:
BREAKING: S&W M&P 15/22 Rifles BANNED at Appleseed Events Due to Run Away and Out of Battery Firing - The Firearm BlogThe Firearm Blog
I think it is rare, but something that S&W really needs to address. Someone has already been injured and required medical care.